The present application relates to the control of superconducting current flows. The present invention finds particular application in conjunction with a solid state superconducting power switch based on the transition of the material between superconducting and normal states and will be described with particular reference thereto. It is to be appreciated, however, that the invention will find further application in superconducting control circuitry.
Superconducting materials typically have a superconducting operating region defined by temperature T, magnetic flux density B, and current density J. Outside the superconducting operating region, the material operates in a normal, resistive state. For the material to be in its superconductive operating region, the current density must be below a critical value J.sup.c, the magnetic flux density below a critical value B.sub.c, and the temperature below a critical value T.sub.c.
A key element in the application of superconducting technology to working devices is the ability to switch the superconducting currents on and off. In an ideal switch, the "closed" state has zero resistance and the "open" state has infinite resistance. The ideal switch also accomplishes this transition in zero time and is completely lossless. In the operating region, a superconducting material has zero resistance and is lossless. In its normal resistive state, superconducting materials have a sufficiently high resistance to define an open state, although the open state does not have the ideal infinite resistance. Further, the transition between the superconductive operating region and the normal resistive state requires a finite amount of time. Thus, with superconducting materials, a "solid state" type switch can be defined merely by moving the superconducting material between its superconducting and normal states.
Heretofore, three methods have been utilized for switching the material between superconducting and resistive states. First, an excess amount of current has been injected into the superconducting material to force the current density to become larger than the critical current density J.sub.c.
A second method, thermal switching, has the advantage of being simple to implement. Several techniques have been used to transfer sufficient energy to the superconductive material to raise its temperature above the critical value T.sub.c and quench the superconductivity. Most commonly, thermal switching is achieved with an auxiliary, resistive heating coil. Other techniques, such as irradiation by a laser source, may be utilized to introduce thermal energy into a limited region of the superconducting medium.
Thermal switching has several disadvantages. To close the switch and return the material to its superconducting condition, the thermal energy must be removed. Such energy removal tends to be energy consumptive and slow.
The third technique is to raise the local magnetic flux density above its critical level B.sub.c. Magnetic quenching is achieved by flooding a region of the superconductor with a magnetic field that is larger than B.sub.c. Most commonly, a magnetic field is generated by an external coil which can be either resistive or superconducting. Magnetic switching is advantageous relative to thermal switching in that it avoids the transfer of heat energy and is faster.
The present invention provides a new and improved magnetic switching method and apparatus.